1. Field of the Invention
The invention relates generally to an apparatus used to measure operating conditions inside a semiconductor wafer processing chamber and, more specifically, to an apparatus for measuring pedestal temperatures by detecting light emissions from a modular component.
2. Description of the Background Art
The materials and processes used to process a semiconductor wafer in a semiconductor wafer processing system are highly sensitive to temperature changes. Should these materials be exposed to excessive temperatures or gradients resulting from poor heat stabilization or transfer, yield or performance of the end product wafer is compromised. For example, if process temperatures are not properly controlled, processes such as etch selectivity and deposition reactions are compromised, along with a degradation in process uniformity. As a result, the circuitry formed upon the wafer are defectively constructed and/or suffer unacceptable variations between devices.
A substrate support, or pedestal, residing in a semiconductor wafer processing chamber is in intimate contact with the wafer and serves both as a heat sink and substrate support. Most pedestals are cooled by conduction to a liquid cooled base where a constant flow of coolant removes excess heat. It has been found that measuring the temperature of the pedestal provides a good indication of the wafer temperature. As such, different means of measuring the pedestal temperature are known in the prior art. One method of determining pedestal temperature is to measure the temperature of the coolant at an outlet of the base. Unfortunately, this method is limited in that the measured temperature is neither an accurate nor timely measure of the pedestal temperature. The coolant temperature in the base is measured after the coolant has passed through several interfaces, conduits, and heat sinks. Use of such unreliable, delayed data (i.e., as a temperature control feedback system parameter) renders wafer temperature stabilization difficult.
A second technique employs a temperature sensitive phosphor sensor. Phosphors used for temperature measurement typically can be grouped into two different categories. The first group emits a fluorescent glow dependent upon the temperature of the phosphor. The second group emits a glow activated by a predetermined impulse of energy, i.e., an exiting pulse of light, and following the activation, exhibit a decay in fluorescence that can be correlated to the temperature of the material. The sensor is mounted inside the chamber with a fiber optic cable which can be either attached to or viewed remotely from the sensor. The fluorescent glow xe2x80x9csignalxe2x80x9d is then transmitted by the fiber optic cable for monitoring purposes. However, as such systems are hard-mounted to the pedestal, the harsh environment within the chamber often contributes to a short service life. Once the probe is damaged, or conversely, if the pedestal suffers a problem requiring service, both the pedestal and probe must be replaced. This contributes to undue expense and unwelcome down time for system maintenance.
A third technique involves diffused reflectance spectroscopy. A wavelength of light reflected from the wafer surface is chosen for monitoring. A spectrometer measures the level of backscattering of the chosen wavelength (with specific energy and related temperature characteristics) to derive a wafer temperature. However, this approach is susceptible to erroneous measurements from energy received from other xe2x80x9chotxe2x80x9d surfaces proximate the measuring equipment.
A fourth technique attempts to measure the pedestal temperature directly during wafer processing by placing thermocouple probes in contact with the pedestal. However, thermocouple probes placed in direct contact with the pedestal are unfortunately subjected to RF interference from a plasma used to process the wafer. As such, it is difficult to obtain an accurate temperature measurement by this approach as well. Thermocouple probes are also cumbersome to install and secure to a pedestal. For example, a small, narrow bore must be formed in the pedestal which is difficult to manufacture. Additionally, as the thermal couple is typically cemented or otherwise adhered to the pedestal at the bottom of the bore, replacement of the thermal couple requires extensive rework and down time.
Therefore, there exists a need in the art for an apparatus that can provide an accurate, real-time measurement of pedestal temperature with a relatively simple and rugged, modular design to withstand a processing chamber environment and facilitate rapid servicing or replacement.
The disadvantages heretofore associated with the prior art are overcome by an apparatus for measuring pedestal temperature in a wafer processing system. The inventive apparatus for measuring a temperature of a pedestal comprises a photoluminescent material disposed on a modular plug. The photoluminescent material decays in luminescence after being exposed to an excitation signal. The rate of decay is indicative of the temperature of the support assembly. The plug is removably disposed in a substrate support assembly to measure the temperature thereof.
The plug is threaded into a plate of the support assembly, thus allowing the plug to be easily removed when either the plug or support assembly requires service. Additionally, the plug is calibrated prior to installation, independent of the probe used to trigger and measure the response. This allows for improved serviceability and the ability to cost effectively retrofit existing processing systems.